Here’s the chart of each of the series per year. Each colour is a different series. If you hover over a section of a bar, it’ll display the series name and how many letters are included from that year.
You can zoom in by clicking and dragging the mouse around an area.
This is my attempt to make sense of the State Papers hierarchy, and to get a sense of the size of each part. The nested boxes can be clicked, which takes you in a further level. At the most zoomed-in level, I’ve listed the top 100 letter-authors for each series. Hovering over a box will give more detail about the name of the person or series.